1. Technical Field
Example embodiments of the present invention relate to a Long Term Evolution (LTE)-Advanced system, which is next-generation mobile communication being standardized by the 3rd Generation Partnership Project (3GPP), and more particularly, to a channel management method according to a radio channel state and a channel management method of efficiently performing retransmission (hybrid automatic repeat request (HARQ)) and discontinuous reception (DRX) control of a terminal in a structure in which communication between a base station and the terminal is performed using a plurality of carriers.
2. Related Art
3GPP, which is a mobile communication standardization organization, has developed an LTE system standard as a next-generation mobile communication standard. Also, the development of the LTE-Advanced system standard as an extension of an LTE standard is under way to satisfy International Mobile Telecommunication (IMT)-Advanced system requirements proposed by International Telecommunication Union-Radio communication Sector (ITU-R).
While the LTE standard supports a maximum radio bandwidth of 20 MHz for mobile communication, the LTE-Advanced standard uses bandwidth aggregation (carrier aggregation) technology to support a maximum bandwidth of 100 MHz. Thus, in the LTE-Advanced standard, the bandwidth of 100 MHz is divided into component carriers (CCs), each of which has a maximum bandwidth of 20 MHz, and a base station may communicate with a terminal by simultaneously using a plurality of carriers.
The terminal, which supports multicarrier communication, uses a plurality of radio channels in a broadband. The wider a frequency bandwidth for use in communication, the higher power consumption. Therefore, the terminal of the LTE-Advanced system activates and uses multiple carriers only when traffic is large, and performs communication using only a single carrier when traffic is small.
FIG. 1 is a conceptual diagram illustrating a general procedure in which a base station configures and activates carriers for a terminal in a multicarrier environment.
Referring to FIG. 1, the LTE-Advanced system controls a configuration procedure 110, an activation procedure 120, and a deactivation procedure 130 into which a multicarrier management step is divided. Although a maximum of five CCs may currently exist in the LTE-Advanced standard, an example in which three CCs (downlink (DL) CC #1 141, DL CC #2 142, and DL CC #3 143) are used as DL CCs is shown in FIG. 1.
When the terminal is connected to the base station that provides multicarrier communication, a multicarrier communication environment is constructed using the configuration procedure. The base station transmits a configuration message to cause the terminal to store configuration information regarding multicarrier communication and prepare carrier communication.
When traffic is low, the terminal communicates with the base station by use of only single carriers (DL CC #1 of FIG. 1 and uplink (UL) CC #1 (not shown)). In a state in which the single carrier is used, the used carrier is referred to as a primary carrier or a primary cell. A radio channel state of the primary carrier is measured in a short cycle and managed as a state in which stable communication is possible.
When traffic of the terminal increases, the base station uses the activation procedure to cause the terminal to actually use a plurality of CCs. A carrier to be activated for the multicarrier environment is referred to as a secondary carrier or a secondary cell of which a radio channel state is managed at a lower level than that of the primary carrier.
The term “carrier” used in the LTE-Advanced system refers to a medium for transmitting a radio channel in a specific frequency band for wireless communication by the base station, and hence has the same meaning as a mobile communication cell. In other words, the terminal of the present invention may perform communication by simultaneously receiving radio channels transmitted in a plurality of cells.
In order to activate the secondary carrier, the base station transmits an activation message to the terminal. Upon receipt of the message, the terminal activates the secondary carrier and uses the activated secondary carrier in communication.
Accordingly, a large volume of data is transmitted and received by multiple carriers, and a data transmission rate is increased. If traffic with the terminal is decreased, the base station transmits a deactivation message to cause the terminal to use only a single carrier. DL and UL CCs may be independently configured/released and activated/deactivated. Also, the DL and UL CCs may be simultaneously activated/deactivated by one message.
When multicarrier communication is performed, data is mapped and transmitted according to each CC. A scheduler of the base station divides data to be transmitted in units of CCs, and the divided data is allocated and transmitted according to each CC. Transmitted data is retransmitted using the same CC.
Under an assumption of FIG. 1, one data block is assigned to DL CC #1. If a data reception error occurs after initial transmission, retransmission (or HARQ) should be performed using DL CC #1. When DL data is transmitted, the data is allocated to a data channel (that is, a physical downlink shared channel (PDSCH)), and demodulation information about the data channel is indicated and transmitted on a control channel (that is, a physical downlink control channel (PDCCH)).
The control channel may be transmitted by the same CC as the data channel or a different CC. If CC positions are different from each other, a position of the data channel is indicated using a carrier indicator (or a carrier indicator field (CIF)) within the control channel.
If traffic between the base station and the terminal increases and a radio channel state of some CCs becomes bad in a state in which multiple carriers are used, the number of data reception errors of a receiver increases and the number of retransmissions increases. In particular, if a radio channel state of a CC becomes bad in a structure in which retransmission is performed by only the CC used during initial transmission as in the LTE-Advanced system, there is a problem in that the number of reception errors rapidly increases.
Also, there is a problem in that data transmitted once is retransmitted until reception succeeds during a maximum number of retransmissions, regardless of a radio channel state. This procedure leads to unnecessary retransmission operations and power consumption of the terminal. Further, there is a problem in that the waste of radio resources and the degradation of base station performance occur because other terminals may not use resources due to unnecessary retransmission.
FIG. 2 is a conceptual diagram illustrating a power consumption reduction (DRX) operation of the terminal in a mobile communication system.
Referring to FIG. 2, the terminal is controlled in a period divided into an on-duration period 201 in which an operation of receiving a DL control channel transmitted from the base station is performed and a sleep period 202 in which an operation of stopping a reception operation and reducing power consumption is performed.
If the base station does not transmit a control channel to the terminal in an on-duration mode, the terminal is switched to a sleep mode by determining that it is not necessary to receive data. If the control channel is received in the on-duration mode, a wakeup state is maintained for a predetermined time by use of an inactivity timer. A cycle in which the terminal performs an on-duration operation is indicated as a DRX cycle 203. The DRX cycle is divided into long DRX and short DRX. In the long DRX, it is possible to minimize power consumption because a data reception cycle of the terminal is long.
In the mobile communication system of the multicarrier structure, all carriers use the same DRX operation procedure. All activated carriers perform the on-duration operation when one carrier is in the on-duration mode, and also the sleep state is equally applied to all carriers.
In order to maintain the DRX operation of each carrier, a DRX-related timer is managed according to each carrier. If one carrier performs a DL control channel reception operation by the timer, all carriers are put in a wakeup mode, thereby performing the same wakeup operation.
On the other hand, a secondary-carrier deactivation procedure and a DRX procedure should be stably controlled so that states managed by the base station and the terminal are consistent with each other if the secondary carrier is configured and operated in the multicarrier environment. When the deactivation procedure is performed, a HARQ retransmission procedure to be performed by the terminal should be managed according to a deactivation state.